Vitamins and supplements are regulated

But before we look at how the science works, it helps to understand how 'medicines' are regulated in Australia.

In a nutshell, not all medicines undergo the same level of scrutiny or oversight. So even though vitamin C might sit next to a cold and flu medication on the shelf, they don't necessarily undergo the same scrutiny.

Complementary medicines and supplements, which include vitamins, minerals, herbal, aromatherapy and homeopathic products, are in a different category to more conventional medicines, such as prescription drugs or over-the-counter medications.

In Australia, all medicines are regulated by the Therapeutic Goods Administration — under which drugs can be categorised as registered (AUST R) or listed (AUST L).

Registered medicines, including prescription medications (e.g. antibiotics) and over-the-counter medications (e.g. painkillers), are "assessed by the TGA for quality, safety and efficacy".

Most complementary medicines are listed, meaning that while they have to meet some safety and hygiene standards, they don't have to meet the same strict efficacy or effectiveness standards that registered medicines do.

This means complementary medicines might not come with the same effectiveness as your conventional medicines.

As far as protection from colds, taking the supplement hasn't been shown to have any kind of preventative effect. Some studies have shown, however, that if you take it early enough in your sickness, then it can reduce symptoms by up to a day and a half.

Adults only need about 30mg of vitamin C per day — and taking any more than 1g can lead to stomach pains or diarrhoea.

"If you're taking multiple different products with similar ingredients, you can easily go over the recommended limit," explains Nerida Packham, pharmacist and Medicines Line Team lead at NPS MedicineWise.

"Remember to look at the active ingredients."

Zinc

What it is: Zinc is a naturally occurring element, which we only need very tiny amounts of to keep our bodies ticking over.

Cold and flu: Similar to vitamin C, can reduce symptoms by about a day.

Possible side effects: Taking too much can have a negative impact on your immune system.

Echinacea

What it is: Echinacea is a plant, and different remedies will use different parts of the plant in varying amounts.

Cold and flu: The evidence is unclear.

Possible side effects: May cause rashes in kids, trigger allergies for asthmatics, interact with other drugs.

Echinacea is often sold as an "immune-boosting" supplement. But this is even harder to pin down the evidence for than vitamin C or zinc — it's a plant, and different remedies incorporate different parts and different amounts.

There also isn't a particular known active ingredient, which can make it tricky to do quality experiments on.

There are a handful of safety concerns associated with the plant, too. It can bring out rashes in children, trigger allergies in asthma patients and — like many complementary medicines — interact with other drugs.

"It's broken down in the liver, so there is a low risk of it interacting with other medications which are processed there — that includes blood thinners, antidepressants, and some antibiotics," Ms Packham says.

Garlic

Image While garlic is a natural herb, it can have some drug interactions.(Unsplash: Mike Kenneally)

What it is: A natural herb that's also sold in supplement form.

Cold and flu: Its impact on preventing or treating cold symptoms is unclear.

Possible side effects: May interact with other drugs, depending on the amount you're taking.

Garlic has long been thought to help with the symptoms of colds and flu.

"There aren't enough clinical trials to provide any solid evidence that it can either prevent colds or treat symptoms," Professor Day says.

Surprisingly, even though garlic is a natural herb, it can have some drug interactions when it's taken in a high amounts in supplement form. These drugs include blood thinners and antivirals.

Don't be shy to ask for advice

Complementary medicines are growing in popularity and you shouldn't feel embarrassed about asking any sorts of questions about them from a health professional.

So, what should you do when you feel a cold or flu coming on?

"For a headache, you would take analgesics so, most commonly, paracetamol. If you want to not be dripping then you can take an antihistamine," he says.

He also emphasises that it's important to get plenty of rest, and stay home if at all possible, as this stops the virus from spreading.

"Just look after yourself a bit more than you normally would. If you're a regular exerciser, then you don't want to be thrashing yourself in the grip of a virus. Take care of yourself, rest, and get plenty of fluid."

This is general information only. For detailed personal advice you should see a qualified medical practitioner who knows your medical history.